Our broad objective is to understand the mechanisms by which the nerve growth factor (NGF) signal is propagated from the axon terminal to the cell body. NGF retrograde signaling is critical for the survival, differentiation, and maintenance of certain types neurons. Disrupted NGF retrograde transport was reported to contribute to the loss of the basal forebarin cholinergic (BFC) neurons in the brains of patients with Alzheimer's Disease or Down's Syndrome. This project will use advanced imaging techniques to directly visualize NGF transport in live neruons in real time. We focus on exploring dynamic features of NGF transport in normal and Down's Syndrome mice. The aims are: 1. Characterize the movement of NGF-containing endosomes in axons and define their pausing mechanism(s), by using quantum dot conjugated NGF to track endosomal movements with nanometer resolution. 2. Determine whether NGF-lacking endosomes are present, whether they are relevant for NGF signaling, and whether there are alternative signaling pathways independent of endosomal transport, by marking the NGF-lacking endosomes with photo-activatable green fluorescence proteins that are fused to the C-terminal of TrkA receptor. 3. Identify the abnormal features of disrupted NGF transport in Down Syndrome mouse neurons, by characterizing individual features of transport dynamics, which inlcude the average speed, the moving speed, the pausing duration, and the pausing frequency. 4. Determine how amyloid precursor protein overexpression leads to the abnormal NGF retrograde transport in Down Syndrome mouse by examining how overexpression of amyloid precursor protein in DS mice might cause defective structural or axonal features that lead to disrupted NGF transport. Achieving those aims will increase our understanding of how NGF signal is propagated in normal and degenerative neurons. More broadly, those studies will contribute to elucidate the pathogenesis of Alzheimer's disease and Down syndrome.